as opposed to a Y-junction, which also has three arms but without this clear distinction of two arms forming one road and the third being a different road.
While some T junctions are controlled by traffic lights, most rely upon drivers to obey agreed rules. In the above image, the vehicle travelling on the horizontal bar of the "T" has right of way; the vehicle approaching the "major" road must allow it to pass before joining the flow of traffic. In this example traffic drives on the left and turning to the right, both to and from the major road is not allowed, as is clear from the double line.